18OSO4 and 18OH2O as prospective indicators of elevated arsenic in the Goose River ground-watershed, Maine

Anomalous geogenic arsenic occurs in drinking water from the Goose River crystalline ground-watershed in mid-coastal Maine. Isotope investigations were useful in understanding release areas of arsenic into affected water wells. The isotope composition of sulfate associated with probable arsenian pyrite oxidation is described. Correlation of '¹⁸O_SO4 enrichment [+4.57 to +7.46‰ Vienna Standard Mean Ocean Water (VSMOW)] is discussed with specific and recurring areas of elevated arsenic (10-52 µg l^-1). Although arsenic concentrations were highly variable over 2 years per well, '¹⁸O_SO4 values were always consistent and suggested a specific and consistent risk for elevated arsenic occurrences for each well. The '¹⁸O values in the water molecule (-12.07 to -8.81‰ VSMOW) and the '¹⁸O_SO4 values may serve as prospective indicators of prominent zones of aeration at depth in discrete fracture zones. The '¹⁸O values in the water molecule and sulfate ion appear to indicate that more than 60% of O₂ incorporated into the SO₄^2- ion are from dissolved oxygen and belong to distinct fractured areas. These aeration zones or oxidation fronts, as outlined by oxygen isotopes, are sentinels for high arsenic risk in groundwater.     

Structure of gas flows in Z And in quiescence and during outbursts

A new mechanism is proposed to account for transitions between the quiescent and active states of symbiotic stars. A numerical study of the gas dynamics of the flows in the symbiotic star Z And shows that even small variations in the velocity of the wind from the cool giant can abruptly change the flow structure near the hot component. Such changes alter the accretion regime as the wind velocity increases: disk accretion makes a transition to accretion from the flow. Our calculations indicate that the accretion rate increases by a factor of several tens over a short time interval (～0.1 of the orbital period) during the rearrangement of the flow, when the accretion disk is destroyed.     

Color redshifts and the age of the stellar population of distant RC radio galaxies

BV RI data are presented for the majority of steep-spectrum objects in the RC catalog with m _R <23.5^m. Previously developed programs are applied to these data to estimate the redshifts and ages of the stellar systems of the host galaxies. Applying this program to the color data (BV RI JHK) for distant radio galaxies with spectroscopic redshifts indicates that this approach provides accurate estimates of the redshifts of such radio galaxies, close to those obtained using field galaxies (～20%). The age estimates are much less trustworthy, but a lower limit to the ages of objects that are not very distant (z<1.5) can be determined with certainty. We have identi fied several galaxies whose formal ages exceed the age of the Universe at the corresponding z in simple Cold Dark Matter models for the Universe. The possibility of using such objects to elucidate the role of “dark energy” is discussed. This paradox disappears in models with cosmological constants (Λ terms) equal to 0.6–0.8.     

Ductile Thrusting Recorded by the Garnet Isograd from Blueschist-Facies Metapelites of the Ile de Groix, Armorican Massif, France

Mineral assemblages in the blueschist-facies metapelites fromthe Ile de Groix (Armorican Massif, France) permit the distinctionof two main units. The Upper Unit is characterized by: (1) highmodal proportions of garnet; (2) larger grain size; (3) therarity of graphite-bearing layers; (4) a single, although composite,foliation S₁. A Lower Unit is defined by: (1) low modal proportionsof garnet; (2) smaller grain size; (3) an abundance of graphite-bearinglayers; (4) a pervasive crenulation cleavage S₂. In the UpperUnit, coexisting garnet and chloritoid are more magnesian andless manganiferous than in the Lower Unit. The differences inmodal proportions and chemistry of coexisting minerals reflectdifferent P–T conditions. The P–T history of theblueschist-facies metapelites is estimated using a simplifiedpetrogenetic grid in the NFMASH system and thermodynamic calculations,which suggest peak P–T conditions at about P = 16–18kbar, T = 450–500°C and P = 14–16 kbar, T =400–450°C in the Upper and Lower Units, respectively.Peak P–T conditions were followed by a nearly isothermaldecompression for both units at slightly different temperatures(of the order of 50°C). The contact between the two units,i.e. the garnet isograd, is interpreted as a greenschist-faciesductile thrust. Thrusting of the higher-grade unit, i.e. theUpper Unit, over the Lower Unit occurred after the high-pressureevent, i.e. during the exhumation of both units. The observedsuperposition of higher-grade rocks over lower-grade rocks arguesagainst models where the exhumation history is entirely controlledby crustal-scale vertical shortening (i.e. extension). KEY WORDS: Armorican Massif; blueschist facies; Ile de Groix; metapelites; P–T path; garnet isograd     

Cordieritite and Leucogranite Formation during Emplacement of Highly Peraluminous Magma: the El Pilon Granite Complex (Sierras Pampeanas, Argentina)

Cordieritites and highly peraluminous granites within the ElPilón granite complex, Sierras Pampeanas, Argentina,were emplaced during a medium-P, high-T metamorphic event duringthe initial decompression of a Cambrian orogen along the southwesternmargin of Gondwana. Very fresh orbicular and massive cordierititebodies with up to 90% cordieritite are genetically associatedwith a cordierite monzogranite pluton and a larger body of porphyriticgranodiorite. The petrogenesis of this association has beenstudied using petrographical, mineralogical, thermobarometric,geochemical, geochronological and isotope methods. The graniticmagmas were formed by anatexis of mid-crustal metamorphic rocksformed earlier in the Pampean orogeny. The cordieritites appearat the top of feeder conduits that connected the source regionlocated at     

Mantle Sources and the Highly Variable Role of Continental Lithosphere in Basalt Petrogenesis of the Kerguelen Plateau and Broken Ridge LIP: Results from ODP Leg 183

Ocean Drilling Program (ODP) Leg 183 was designed to investigatethe origin and evolution of the large igneous province composedof the Kerguelen Plateau and Broken Ridge. Of the eight sitesdrilled, basalt was recovered from seven, five on the plateauand two on Broken Ridge. We present results from four of thesesites, 1136, 1138, 1141 and 1142. Although this large igneousprovince is interpreted as being derived from the Kerguelenmantle plume, the geochemical characteristics of basalt fromsome parts of the province indicate a role for continental lithosphere.The 118–119 Ma basalt flows recovered in the SouthernKerguelen Plateau (Site 1136) have a more subtle continentalsignature than shown by basalt at Leg 119 Site 738. A continentalsignature is absent in the 100–101 Ma tholeiitic basaltsat Site 1138 in the Central Kerguelen Plateau (CKP); their age-correctedNd–Sr–Pb isotopic values and incompatible elementratios are similar to those estimated for primitive mantle.These flows may represent a major mantle source in the Kerguelenstarting-plume head. The 20 basalt units identified are a productof magma chamber replenishment, fractional crystallization,and resorption of crystallizing phases. The topmost unit, Unit1, is a dacite that evolved from a basalt magma similar to thoserepresented by Units 3–22; unlike the basalts the dacitemagma was probably influenced by continental material. MiddleCretaceous (     

The Fish Canyon Magma Body, San Juan Volcanic Field, Colorado: Rejuvenation and Eruption of an Upper-Crustal Batholith

More than 5000 km³ of nearly compositionally homogeneous crystal-richdacite (     

Crustal Evolution in the SW Part of the Baltic Shield: the Hf Isotope Evidence

The results of a laser ablation microprobe–inductivelycoupled plasma mass spectrometry Lu–Hf isotope study ofzircons in 0·93–1·67 Ga rocks from southNorway indicate that early Proterozoic protoliths of the BalticShield have present-day ¹⁷⁶Hf/¹⁷⁷Hf     

Quantitative analysis of variations in depositional sequence thickness from submarine channel levees

Abstract Thickness variations across‐levee and downchannel in acoustically defined depositional sequences from six submarine channel‐levee systems show consistent and quantifiable patterns. The thickness of depositional sequences perpendicular to the channel trend, i.e. across the levee, decreases exponentially, as characterized by a spatial decay constant, k. Similarly, the thickness of sediment at the levee crest decreases exponentially down the upper reaches of submarine channels and can be characterized by a second spatial decay constant, λ. The inverse of these decay constants has units of length and defines depositional length scales such that k^?1 is a measure of levee width and λ^?1 is a measure of levee length. Quantification of levee architecture in this way allowed investigation of relationships between levee architecture and channel dimensions. It was found that these measures of levee e‐folding width and levee e‐folding length are directly related to channel width and relief. The dimensions of channels and levees are thus intimately related, thereby limiting the range of potential channel‐levee morphologies, regardless of allocyclic forcing. A simple sediment budget model relates the product of the levee e‐folding width and e‐folding length to through‐channel volume discharge. A classification system based on the quantitative downchannel behaviour of levee architecture allows identification of a ‘mid‐channel’ reach, where sediment is passively transferred from the through‐channel flow to the levees as an overspilling flow. Downstream from this reach, the channel gradually looses its control on guiding turbidity currents, and the resulting flow can be considered as an unconfined or spreading flow.